The present invention relates generally to the field of spinal implant systems, and particularly systems of the type that employ vertebral fixation elements engaged to elongated members spanning the spine. More particularly, the invention concerns a connector that provides variable angle adjustability between the bone fixation element and the elongated member.
Several systems have been developed for the use in correcting and stabilizing spinal deformities and facilitating spinal fusion. In one such system, an elongated member, typically in the form of a bendable rod, is longitudinally disposed adjacent the vertebral column and is fixed to various vertebrae along the length of the column by way of a number of fixation elements. A variety of fixation elements are used, such as hooks, bone bolts and screws, each configured to engage a specific portion of a vertebra.
An example of a spinal fixation system of this type is the TSRH.RTM. Spinal System of Sofamor Danek Group, Inc. In this product, spinal hooks and bone screws are engaged to a spinal rod by way of eyebolts. The eyebolts and associated clamping nut provide a three-point shear clamp that positively locks the hook or screw element to the spinal fixation rod.
In its early years, the TSRH.RTM. Spinal System only permitted fixed orientations of the spinal hook or bone screws with respect to the rod. In other words, the angle and attitude between the fixation element and spinal rod could not be varied. In order to address this limitation, the TSRH.RTM. variable angle screw was developed. The details of this screw are described in U.S. Pat. No. 5,261,909, owned by the Assignee of the present invention. In general terms, this variable angle screw utilized a separate washer that was engaged over a clamping eyebolt. The washer and a face of the head of the fixation element included interdigitating splines that permitted variable angular orientations between the fixation element and the washer. A nut is used to clamp the assembly together, thereby fixing the specific angular relationship between the bone engaging fastener and the spinal rod. While this variable angle screw revolutionized spinal fixation, and particularly rod-based systems, it too has certain limitations. For instance, the angular orientations that can be accomplished are limited to the number and arrangement of interdigitating splines. In other words, the TSRH.RTM. variable angle screw, as with the variable angle screws developed subsequently, are limited to a finite number of angular positions within the range of motion. These finite angular positions have, at times, caused surgeons to be forced to manipulate either the vertebra and/or the implants in order to properly mate the fixation element to the variable angle connector engaged to the spinal rod.
Another limitation of the original TSRH.RTM. variable angle screw is that it does not accommodate variable height positions along the axis of the screw. In other words, the distance between the spinal rod and the vertebral body cannot be varied mechanically, but can only be varied by increasing or decreasing the depth that the screw is threaded into the vertebral body. In the case of spinal hooks, this latter capability is not present.
Contemporaneous with the variable angle screw, Sofamor Danek, developed the GDLH.RTM. Spinal Fixation System. This GDLH.RTM. System includes spinal fixation elements, such as pedicle hooks, that incorporate an elongated stem. The GDLH.RTM. System also includes a rod connector that receives the elongated stem and permits variable height or axial positions of the fixation element relative to the connector and the spinal rod. In this system, the portion of the elongated shank of the fastener extending beyond the connector can be severed to reduce the overall profile of the construct. While the GDLH.RTM. System addresses the difficulty of height variability, the system does not include a variable angle capability.
Thus far, there is no known spinal system that provides a reliable connection between a fixation element and an elongated member spanning the spine, while permitting variable angular and height, or axial, orientations between the two elements. Likewise, there is no known system that combines these degrees of freedom of relative movement in an infinitely variable fashion. Such a system would significantly eliminate the problem of "fiddle factor" in which a surgeon must manipulate either the spine or the instrumentation in order to complete the construct.